(1) Field of the Invention
This invention relates to a testing chart for acquiring a correction value for use in discharge correction in an inkjet printing apparatus which discharges ink droplets to form images on a printing medium, to a correction value acquiring method for an inkjet printing apparatus, and to an inkjet printing apparatus.
(2) Description of the Related Art
An inkjet printing apparatus discharges ink droplets from an inkjet head having a plurality of nozzles to printing paper to form images thereon. At this time, the number of nozzles which simultaneously discharge ink droplets increases or decreases according to an image to be formed, and this causes variations in the pressure loss in the inkjet head and in the load of a drive circuit. As a result, ink droplet discharge characteristics of the inkjet head vary from time to time, causing a reduction in printing quality.
Under the circumstances, the following apparatus have been proposed as techniques for solves such problem.
A first apparatus has a detector for detecting the number of nozzles which discharge ink droplets among a plurality of nozzles constituting an inkjet head. In response to the number of nozzles detected by this detector, a drive signal for discharging ink droplets from the inkjet head is adjusted with a correction value (see Japanese Unexamined Patent Publication H5-116342, for example).
A second apparatus has correction tables storing beforehand amounts of correction of a drive signal matched with numbers of nozzles which discharge ink droplets, and an image data counter for counting the number of nozzles which discharge ink droplets at the same time. The apparatus reads a correction table corresponding to the number of nozzles counted by this image data counter, and adjusts the discharge timing of ink droplets with the amount of correction matched therewith (see Japanese Unexamined Patent Publication No. 2011-148287, for example).
However, the conventional examples with such constructions have the following problems.
That is, the conventional apparatus, which require correction values corresponding to numbers of nozzles discharging at the same time, have a problem that the correction values cannot be obtained easily.
Incidentally, in order to obtain the correction values, it is conceivable to use a characteristic measuring device for the inkjet head (see Japanese Unexamined Patent Publication No. 2011-101870, for example). However, since it is necessary to take the trouble of using such characteristic measuring device, it is after all not easy to acquire the correction values, and besides there is a problem of taking a long time until a correction value is obtained. Moreover, since the conditions at the time of measurement differ from actual printing conditions (e.g. types of printing paper), even the correction value acquired in this way will not necessarily bring satisfactory correction results. Further, in order to meet the latest requirement for high resolution, the inkjet head may be constructed by combining a plurality of head modules, for example. Since only a correction value for each individual head module is obtained from the characteristic measuring device, there is a problem of being unable to carry out correction accurately for one inkjet head whose operation relies on the combination of these head modules.